A new transplantable mammary tumor in the mouse will be used as a model system for the study of estrogen and progesterone receptors. This tumor model has cytoplasmic and nuclear estrogen receptors which are similar to other estrogen sensitive tissues. An exchange assay for both cytoplasmic and nuclear estrogen receptors has been established and will be used as a marker for estrogen action, and the ability of progesterone to act as an estrogen antagonist will be tested. The mechanisms by which triphenylethylene derivatives, such as Nafoxidine and Clomid, act as estrogen agonists (not antagonists as is generally the case) will also be examined. These hormone-induced responses and receptor characteristics will be compared to those observed in normal mammary tissue.